There are 10 kinds of isomers of dimethylnaphthalene (DMN), which are classified into three groups according to positions of 2 methyl groups on naphthalene rings: a group of α,α-isomers including 1,4-DMN, 1,5-DMN, and 1,8-DMN; a group of α,β-isomers including 1,2-DMN, 1,3-DMN, 1,6-MDN, and 1,7-DMN; and a group of β,β-isomers including 2,3-DMN, 2,6-DMN, and 2,7-DMN. In general, purification of organic compounds is carried out by a procedure such as distillation, crystallization, and adsorption or by a combination of those. However, the DMN isomers have extremely small differences in melting point and boiling point, so it is difficult to use the simple purification method such as distillation or crystallization. For a separation method for a DMN mixture, there have been conventionally known methods and the like involving separating DMN by crystallization and by means of an adsorbent. There are known a method of separating, especially, 1,4-DMN from a DMN isomer mixture by using as an adsorbent zeolite (see, Patent Document 1), a method of selectively separating 2,6-DMN by using an auxiliary solvent for promoting adsorption and development, a desorbing agent, and zeolite Y (see, Patent Document 2), methods of separating 2,6-DMN in high purity from a feedstock oil containing 2,6-DMN, including using zeolite Y subjected to ion exchange with a cation as an absorbent and using m-xylene as a desorbing agent and a solvent (see, Patent Documents 3 and 4), and the like.
However, in those methods, selectivity of the DMN isomers is not sufficient, and the resultant DMN includes a plurality of isomers. Resins obtained from naphthalene dicarboxylic acid dimethyl ester obtained by subjecting the DMN isomer mixture to oxidation/esterification have insufficient physical and mechanical properties such as heat resistance, mechanical strength, and dimensional stability, and thus cannot be used as raw materials for polyester and the like. In particular, there have not been established a method of separating an α,β-isomer from a DMN mixture containing an α,α-isomer, an α,β-isomer, and a β,β-isomer, for example, a method of separating 1,3-DMN from a mixture of 1,3-DMN, 1,4-DMN, and 2,3-DMN. Thus, investigations have been conducted for a long period of time for attaining industrially useful separation methods by which highly pure DMN is obtained.                Patent Document 1: Japanese Patent Application Laid-Open No. 62-240632        Patent Document 2: Japanese Patent 3157253        Patent Document 3: Japanese Patent Application Laid-Open No. 6-65114        Patent Document 4: Japanese Patent 2641201        